1. Field of the Invention
The present invention relates in general to alternators and in particular to alternators for use in power generators for vehicles.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
It is well known in the art that the magnitude of the current that can be supplied by an alternator when the speed of rotation of the engine, and hence of the alternator rotor, is very high, is limited by the number of turns of the winding. At high speeds of rotation, in order for an alternator to be able to supply a very high current, it is necessary only that the induction winding of the alternator to be made with few turns. However, at low speeds, in order to obtain sufficient current, it is necessary for the winding to have many turns.
Thus, in order to provide alternators that are capable of supplying large currents at high running speeds and at the same time currents of a satisfactory magnitude at low running speeds, the alternator induction windings are subdivided into two or more windings intended for a connection in series with each other at low running speeds and in parallel with each other at other higher speeds. Thus, as can be seen in FIG. 8 of the drawings herein, at low speeds where a large power is required, the generator produces an output essentially as shown by the portion of curve A that is greater than the power requirements shown in curve C. When the rate of rotation of the engine of the vehicle exceeds some predetermined value, the windings of the alternator are now connected in parallel with each other and are equivalent to a single winding with a smaller number of turns compared to the series connection. Under these conditions, the supply of current and power by the alternator is essentially that shown by the second part of the characteristic curve B illustrated in FIG. 8. Again, the supply is greater than the power requirements shown in that range by curve C.
Such a system is shown in U.S. Pat. No. 4,339,704 wherein a first leg of six serially-connected diodes are coupled in parallel with a second leg of six series-coupled diodes between a DC bus having a positive and a negative terminal. There is a first junction between the first and second serially-connected diodes in both the first and second legs, a second junction between the second and third diodes in both of the parallel legs, a third junction between the third and fourth diodes in both parallel legs, a fourth junction between the fourth and fifth diodes in both parallel legs, and a fifth junction between the fifth and sixth diodes in both parallel legs.
A first coil, C1, from an alternator is connected across the first junction of both parallel legs. A first switch, S1, that can be opened and closed is connected between the second junction of the first and second parallel legs. A second coil, C2, of the alternator is connected between the third junction of both of the parallel legs. A second switch, S2, is coupled between the fourth junction of each of the parallel legs, and a third coil of the alternator is connected between the fifth junction between both parallel legs. When both switches are opened and all of the alternator coils are poled in the same direction, the coils are simply in parallel and, as can be seen in FIG. 1, twelve diodes are required to couple power between the negative and positive output buses. However, when both switches are closed, the three alternator coils are connected in series between the positive and negative power output buses and six diodes are required for each phase of the current flowing through the alternator coils. These diodes are power diodes and are quite expensive.
Thus, as can be seen in FIG. 1, a first leg containing diodes D1, D3, D5, D7, D9, and D11 are coupled in series between the positive and negative power buses. A second parallel leg having diodes D2, D4, D6, D8, D10, and D12 are also serially connected between the positive and negative output terminals. A first current-generating coil, C1, is coupled between the first junction in both legs between the diodes D1 and D3 and D2 and D4. A first switch, S1, that can be opened and closed is coupled between a second junction between diodes D3 and D5 in one leg and D4 and D6 in the second leg. A second coil, or winding C2, is coupled to the junction between diodes D5 and D7 and D6 and D8. A second switch, S2, is coupled between the junctions of diodes D7 and D9 and diodes D8 and D10. Finally, a third coil, C3, is coupled between the junction in the first leg between diodes D9 and D11 and between diodes D10 and D12 in the second leg.
When switches S1 and S2 are open, the output of the coils, since they are all poled alike, are coupled in parallel through the twelve diodes D1-D12 to the positive and negative terminal buses.
However, when the switches are closed to place the three coils in series, as shown in FIGS. 2 and 3, the circuit path is rather circuitous. For instance, in FIG. 2, with the coils poled as shown, the current flows from the negative terminal through D12, coil C3, diode D9, switch S2, diode D8, coil C2, diode D5, switch S1, diode D4, coil C1, and diode D1. Of course, by the current having to travel through all of these six diodes and two switches, not only in great amount of heat generated, but there is a great deal of energy loss in the diodes. During the negative half of the cycle of the coils, when they are poled as shown in FIG. 3, the current flows from the negative DC terminal through D11, coil C3, diode D10, switch S2, diode D7, coil C2, diode D6, switch S1, diode D3, coil C1, and diode D2. Although a different path is taken, it is clear that the current again flows through six diodes and two switches, again creating a considerable energy loss and requiring very expensive diodes to carry the current necessary.
Thus, twelve diodes are required for parallel operation of three alternator coils and six diodes are required for series operation of three alternator coils.
It would be advantageous to have a switching alternator system that could switch the alternator coils from series to parallel with as few diodes as possible.